Bottom Line:
Contrary to expectations, our first experiment revealed the opposite effect: cTBS enhanced conscious vision relative to a sham control.No significant effects of cTBS on MEG measures were observed, although the results provided weak evidence for potentiation of event related desynchronisation in the β band.We speculate that gating-by-inhibition in the visual cortex may provide a key foundation of consciousness.

Affiliation: Cardiff University Brain Research Imaging Centre, School of Psychology, Cardiff University, Cardiff, Wales, United Kingdom.

ABSTRACTThis series of experiments investigated the neural basis of conscious vision in humans using a form of transcranial magnetic stimulation (TMS) known as continuous theta burst stimulation (cTBS). Previous studies have shown that occipital TMS, when time-locked to the onset of visual stimuli, can induce a phenomenon analogous to blindsight in which conscious detection is impaired while the ability to discriminate 'unseen' stimuli is preserved above chance. Here we sought to reproduce this phenomenon using offline occipital cTBS, which has been shown to induce an inhibitory cortical aftereffect lasting 45-60 minutes. Contrary to expectations, our first experiment revealed the opposite effect: cTBS enhanced conscious vision relative to a sham control. We then sought to replicate this cTBS-induced potentiation of consciousness in conjunction with magnetoencephalography (MEG) and undertook additional experiments to assess its relationship to visual cortical excitability and levels of the inhibitory neurotransmitter γ-aminobutyric acid (GABA; via magnetic resonance spectroscopy, MRS). Occipital cTBS decreased cortical excitability and increased regional GABA concentration. No significant effects of cTBS on MEG measures were observed, although the results provided weak evidence for potentiation of event related desynchronisation in the β band. Collectively these experiments suggest that, through the suppression of noise, cTBS can increase the signal-to-noise ratio of neural activity underlying conscious vision. We speculate that gating-by-inhibition in the visual cortex may provide a key foundation of consciousness.

Mentions:
Contrary to our hypothesis, we observed no discernible effect of the cTBS, compared to sham/iTBS control, upon evoked responses (see Figure 11). The ANOVA indicated that there was neither a significant main effect of site (cTBS vs. control, F(1,22) = 0.31, p = 0.59, d = 0.14, N = 1 outlier excluded) nor a significant interaction between site × time post TBS (F(3,66) = 0.63, p = 0.57). This was reflected by the Bayesian analysis, which supported the hypothesis of an absence of positive effects, but not unequivocally (Bcombined = 0.57, Breplication = 0.728). No time effect was observed in isolation (F(3,66) = 0.83, p = 0.48).

Bottom Line:
Contrary to expectations, our first experiment revealed the opposite effect: cTBS enhanced conscious vision relative to a sham control.No significant effects of cTBS on MEG measures were observed, although the results provided weak evidence for potentiation of event related desynchronisation in the β band.We speculate that gating-by-inhibition in the visual cortex may provide a key foundation of consciousness.

Affiliation:
Cardiff University Brain Research Imaging Centre, School of Psychology, Cardiff University, Cardiff, Wales, United Kingdom.

ABSTRACTThis series of experiments investigated the neural basis of conscious vision in humans using a form of transcranial magnetic stimulation (TMS) known as continuous theta burst stimulation (cTBS). Previous studies have shown that occipital TMS, when time-locked to the onset of visual stimuli, can induce a phenomenon analogous to blindsight in which conscious detection is impaired while the ability to discriminate 'unseen' stimuli is preserved above chance. Here we sought to reproduce this phenomenon using offline occipital cTBS, which has been shown to induce an inhibitory cortical aftereffect lasting 45-60 minutes. Contrary to expectations, our first experiment revealed the opposite effect: cTBS enhanced conscious vision relative to a sham control. We then sought to replicate this cTBS-induced potentiation of consciousness in conjunction with magnetoencephalography (MEG) and undertook additional experiments to assess its relationship to visual cortical excitability and levels of the inhibitory neurotransmitter γ-aminobutyric acid (GABA; via magnetic resonance spectroscopy, MRS). Occipital cTBS decreased cortical excitability and increased regional GABA concentration. No significant effects of cTBS on MEG measures were observed, although the results provided weak evidence for potentiation of event related desynchronisation in the β band. Collectively these experiments suggest that, through the suppression of noise, cTBS can increase the signal-to-noise ratio of neural activity underlying conscious vision. We speculate that gating-by-inhibition in the visual cortex may provide a key foundation of consciousness.